Field of the Invention
The present invention relates to a boom used in an industrial machine or the like, or more specifically to a structure of a telescopic boom provided to, for example, a mobile crane or the like.
Description of the Related Art
There is conventionally known a telescopic boom assembly including telescopic booms each having a closed section formed by a first frame whose section is substantially U-shaped and a second frame (see Japanese Patent Application Publication No. 2006-21877).
As shown in FIG. 5, a telescopic boom of this type has a first frame 1 whose section is substantially U-shaped and a second frame 2 attached on top of the first frame 1, and forms a closed section with the first frame 1 and the second frame 2.
A small compartment portion 3 is formed at a center portion of the first frame 1, the small compartment portion 3 having an arc section and protruding downward. Small compartment portions 4 and 5 are formed on both sides of the small compartment portion 3, respectively, at positions away from the small compartment portion 3 each by a predetermined distance, the small compartment portions 4 and 5 each having an arc section and protruding outward. The small compartment portions 3 to 5 have the same radius.
Improvement in the buckling strength of the first frame 1 of the telescopic boom is aimed with these three small compartment portions 3 to 5.
Although the above-described telescopic boom aims to improve the buckling strength of the first frame 1 with the three small compartment portions 3 to 5, it is difficult to achieve further improvement in the buckling strength with the structure having the three small compartment portions 3 to 5.